1. Field of the Invention
This invention relates to a printing apparatus for effectuating recording by mixing metered amounts of an ink and a diluting solution and emitting the resulting mixed solution to a recording medium, and a recording solution employed for the printing apparatus. More particularly, it relates to a printing apparatus in which an organic solution or an aqueous dispersion of a resin non-soluble in water is contained in the diluting solution for improving ink fixing performance and emitting stability for prohibiting picture bleeding.
2. Description of the Related Art
Recently, document preparation by a computer, termed desk top publishing, is in widespread use especially in offices. There is also an increasing demand for outputting not only a letter or a figure but also a colored natural picture comparable to a photograph along with a letter or figure. In keeping up therewith, a demand is raised for printing a high-quality natural picture and halftone reproduction has become crucial.
A so-called on-demand printer apparatus, in which ink droplets are ejected responsive to recording signals from a nozzle only when required during printing for recording on a recording medium, such as a film, has also come into widespread use because it lends itself to reduction in size and in cost.
While a variety of methods for discharging ink droplets has been proposed, a method employing piezo-electric element or a method employing a heating element is most popular. The former method consists in applying pressure to the ink by deformation of the piezo-electric element, while the latter method consists in heating and boiling the ink by the heating element for generating bubbles for discharging the ink.
Also, a variety of methods have been proposed for regenerating the half-tone by the on-demand printing apparatus discharging the ink droplets. A first method consists in varying the voltage or pulse width of a voltage pulse applied to the piezo-electric element or the heating element for controlling the size of the ejected droplet for varying the diameter of printing dots for representing the gradation. In the above-described on-demand printing apparatus, one droplet is ejected per voltage pulse.
With this first method, if the voltage or the pulse width applied to the piezo-electric element or the heating element is lowered excessively, the ink ceases to be ejected. Consequently, there is a limitation imposed on the minimum liquid droplet diameter, while the number steps of gradation that can be represented is small and it is extremely difficult to express low density. Thus the method is not suited to print-out of a natural picture.
A second method consists in constituting a pixel by a matrix of 4.times.4 dots, without changing the dot diameter. The gradation is represented using a so-called dither method on the matrix basis.
If each pixel is represented by a 4.times.4 matrix with this second method, density may be represented by 17 steps of gradation. However, if printing is made with the same dot density as that of the first method, the resolution is lowered to one-fourth such that only coarse printing results. Thus the second method also is not suited to printout of natural pictures.
The present inventors have proposed a printing apparatus in which the ink and a diluting solution are mixed together when discharging the ink for varying the density of the ejected ink liquid droplets for controlling the density of printed dots for printing the natural picture without deteriorating the resolution. In the printer apparatus, water admixed or not admixed with surfactants is used as the dilution liquid.
Recently, an increasing demand is raised for suppressing bleeding and improving the ink fixing performance in a picture and discharging stability in a printing apparatus. Such demand cannot be met without difficulties by a printing apparatus employing water admixed or not admixed with surfactants as the above dilution liquid.
In particular, if water admixed with the surfactant is used as the dilution liquid, bleeding of an indefinite or irregular shape tends to be produced on printing on an ordinary paper sheet, thus occasionally detracting from the picture quality, even although the dilution liquid is superior in permeability to the recording medium, such as printing sheet, and hence assures ink fixation.